Muhammad Ali could talk and jive all he liked: his opponent, far bigger than any he had faced in the ring, was pulverising him. During a few feverish months in 1967 Ali was stripped of his heavyweight title, slung into jail and had his boxing license ripped up; all for the crime of refusing to take a single forward step – the step that signalled willing induction into the US army. Ali insisted he had no quarrel with the Viet Cong but much of white America, particularly in the south, did with him. They roared for his blood.

The Man had taken his title, his liberty and his livelihood. And then a machine spat in his face.

It was Murry Woroner, a short, chunky, balding advertising executive from Miami, who was among the first to grasp that marrying fantasy with nascent computer technology was a licence to print money. While Ali's career was being parked in a pallid wasteland for nearly four years, the best years of his fighting life, Woroner's was accelerating from first to fifth. His idea was simple. A fantasy radio boxing tournament to determine the best heavyweight of all time with a twist: the results would be calculated by a second-generation NCR 315 computer, packed with 5k of handmade core memory and the icy dispassion of an implacable neutral. Soon it had 12 million listeners.

A flattering 1968 Sports Illustrated piece, entitled 'And In This Corner .... NCR 315', hailed the tournament as "one of the most astonishing marketing successes in radio history". Woroner, it added, "brought to our wondering ears, via radio and computer, the All-Time Heavyweight Tournament and Championship Fight. He reduced 16 magnificent fighters (from John L. Sullivan to Muhammad Ali) to keypunch perforations, fed them into a National Cash Register 315 computer and let them fight: the bareknucklers v the gloved sluggers, the rigid standers v the dodging dancers, the quick v the dead. From the computer readouts, he produced breathless blow-by-blow broadcasts, peddled the tapes to 380 stations the world around and, after 15 elimination bouts, let it be known last December that Computer Fighter No.004 (Rocky Marciano) had knocked out Computer Fighter No.002 (Jack Dempsey) in the 13th round of the finals."

Woroner, incidentally, was not someone acquainted with hubris. "We could do more than sports," he told Sports Illustrated. "Much more. Wars! Hitler's Germany against the Roman Empire! Napoleon versus Alexander the Great! How about election campaigns? George Washington versus Franklin Roosevelt! Abraham Lincoln against George Wallace! And debates? Socrates takes on Karl Marx! Thoreau against Jean-Paul Sartre! Why not? Why not?"

But before Woroner could solve every pub argument in history, Ali hit him with a $1m lawsuit for defamation. The NCR 315's circuit boards had calculated that Ali would have lost in the quarter-finals to Jim Jeffries – a fighter Ali dismissed as "history's clumsiest, most slow-footed heavyweight." The government had stolen his title, he fumed, and now Woroner was taking his good name.

This being boxing, an accommodation was reached. Woroner offered Ali $9,999 to film a fantasy fight against Marciano and he accepted. He was short of money and options – "I was in the deep-freeze part of my exile and there was no thaw in sight", he confessed in his autobiography – and the Super Fight was born.

THE ROCK RETURNS

23 September 1952: Rocky Marciano knocks out Joe Walcott in the 13th round of their world heavyweight title bout in Philadelphia. Photograph: Corbis Unknown/ Bettmann/CORBIS

By the time Rocky Marciano faced Ali, he had not fought for 13 years. He was 45, balding, and had a bad back. Retirement had been better for his wallet than his waistline: the energy burnt with women other than his wife a timid counter to his binges on rich Italian food and lack of exercise.

Marciano had remained active in other ways following his final fight against Archie Moore in 1956. TV presenting. Product endorsements. Restaurant chain business. Sausage company owner. Even wrestling referee. He tried them all. He kissed the cheeks of made men and did deals with spaccones, who saw him coming from blocks way. Marciano always feared a return trip to the cloying poverty of his upbringing. But $3m in ring earnings, an addict's craving for more and an aversion to paying for anything – he even used wires to avoid putting a dime in public phone boxes – ensured that was unlikely to happen.

Willie Pep, the great featherweight and friend of Marciano, once told how he tried to buy a round while out with Rocky and some wealthy men in a Baltimore nightclub. "I figured I'd take care of the next round," Pep told Marciano's biographer, Everett Skehan. "But then I saw Rocky was squirming under his seat. The next thing I knew he kicked me under the bar." After the pair excused themselves from the group, Marciano told him: 'I'm not gonna spend any money, and I don't want you to spend any. Don't make me look bad, Willie.' Pep remembers: 'Rocky was a tough guy with a buck. He earned it the hard way and was determined to keep it'."

"He had this crazy, crazy need for cash," his accountant Frank Saccone said. "He'd reach in his pocket and pull out checks that were all tattered. I've seen him give away checks for $50,000, $100,000. I'm talking big money. He didn't even associate that with money. To him a check was just a piece of paper. But if he had $40,000 in $10 bills, there was no way he'd give any of that away. He believed in green stuff."

But while there were million-dollar siren calls from promoters, Marciano resisted the temptations to slip the gloves back on. His 49-0 record, with 43 knockouts, remained pristine and unimpeachable. Still, he missed the spotlight and loved risk, which is why the idea of a 'fight' with Ali might have appealed – especially when he knew the dice were loaded. "If you want to live a full life then live dangerously," he wrote in one notebook. "Champs are not (never) supposed to play safe playing against the clock jab and move," in another.

Jab and move wasn't something Marciano ever practised. He was as subtle as a sledgehammer. "His footwork," wrote the Associated Press reporter Whitney Martin, "consists of moving forward in a direct line to a point where he is within cannonading range." Moore was just as blunt: "Rocky didn't know enough boxing to know what a feint was. He never tried to outguess you. He just kept trying to knock your brains out."

It was the Marciano way. He was a modest-sized heavyweight – 5ft 10in tall, around 13 1/2st and with a 68in reach, the smallest of all the heavyweight champions – who powered-up his genetics with a Fordian work ethic, an iron will and a wrecking-ball of a right hand – his "Suzie Q". His left hook was almost as crushing, and one sparring partner described getting hit by a single Marciano blow as equivalent to four from Joe Louis, no mean puncher himself.

The Pulitzer Prize winning writer Red Smith called Marciano "the toughest, strongest, most completely dedicated fighter who ever wore gloves," adding "fear wasn't in his vocabulary and pain had no meaning." While Don Turner, who has worked with the likes of Larry Holmes and Evander Holyfield, still speaks of Marciano with awe. "My first professional trainer was Charley Goldman [the man who guided Marciano]," he says. "We used to sit and talk about Rocky Marciano all the time. He had as much determination as any fighter ever. There was no quit in him at all. He knew what his limitations were and he made up for them by working as hard as any fighter who ever lived.

"Once a boxer starts cheating in training, he can't be great," adds Turner. "Marciano never cheated in training. Out of the thousand or so days that he was champion, I'll bet he was in the gym and working hard for all but 150 of them. If there was a problem in training camp, it was that they had trouble getting sparring partners because Marciano hit them so hard. He wouldn't even take a phone call during the 10 days before a fight. That's how focused his mind was."

Time has not been kind to Marciano's reputation. He is regarded as a second-tier champion. Too small, too crude, too hittable. In his time, though, he was accepted as one of the greats. It helped that he also embodied the post-war American dream: if an undersized son of a poor Italian-American shoemaker could make it, anyone could.

"... And now this all-time heavyweight championship fight is ready to go, and there's the bell and here's [the commentator] Guy LeBow … Rock Marciano, Muhammad Ali/Cassius Clay in this the classic championship fight and uniquely the two only undefeated heavyweight champions of the world. I'm rather staggered by the size propositions. Marciano about 5ft 11, Cassius Clay about 6ft 3, perhaps a little bit more. Marciano is the shortest man Cassius has fought and Marciano is looking at the tallest man he has fought …"

In July 1969, the month that Neil Armstrong took one giant leap for mankind and technology, Marciano and Ali stepped into a gym with blacked-out walls on the north side of Miami and boxed 70 one-minute rounds. They, too, were supposedly guided by computers; marionettes punching and parrying to the whims of the NCR 315.

Like the Fantasy Heavyweight Tournament two years' earlier, the outcome of the Marciano v Ali contest would apparently be based on data collected from 250 boxing experts, who – according to Sports Illustrated – had filled in sheets that took "58 rating 'factors,' ranging from the obvious (speed, susceptibility to cuts, ability to throw a left) to the sublime (hardness of punch, killer instinct, courage)." This data was fed into the computer, which whirred and chugged before finally spitting out its verdict.

The magazine made the process sound rigorous and illuminating. "Woroner or LeBow interviewed every living fighter … with the exception of Gene Tunney, who declined to be involved," it wrote. "From all this they compiled as encyclopaedic an accumulation of boxing trivia and technicalities as anyone had ever put together. They knew how often and where each fighter cut his opponents, where he was cut most often himself, how many punches and what kind he usually threw in a round, what pattern, pace and rhythm he preferred, what blows hurt him most, how many fouls he had committed."

It was largely spin. As Ali later admitted in his autobiography, "there was no computer telling us what to do".

Instead they laboured around the ring, avoiding head punches and mostly tapping at each other's stomachs. A duvet of flab embraced Ali's mid-section, and his jabs contained the spite of a well-fed labrador. Marciano, who had lost 45lbs in case his opponent took liberties, was more serious. But a new toupee, which he believed made him look well-groomed and youthful, further turned this curiosity towards cartoon: he looks like an undersized hoodlum from Dick Tracy.

At one point the pair were exchanging blows when Ali's jab flicked the back of Marciano's head and scooped up his toupee.

"Cut! Cut! Cut the camera," shouted Marciano, "Watch the piece!"

Later he asked his friends: "You don't think he's doing it on purpose?"

"No Rock," his friends assured him. "It's just an accident."

"Well, he'd better start aiming those punches better," Rocky said.

"Rock was really upright about the toupee," Ali's trainer, Angelo Dundee, said. "He had this guy in New York that made his toupees. I remember when he got the first one. Mingia! It was terrible. It looked like a dead cat. I said, 'Rocky, watch out. The thing might get up and run away."

Sadly the scene didn't make the final edit.

The theatre is further enhanced by knowing that the 'blood' from Marciano's cuts to his nose and forehead, which he develops in the fight, is ketchup. Wrote Ali: "My glove never hit his face, his glove never hit mine … the promoter asks me if I can think of some ending, and I plan the one that is actually used: I show Rocky how to hit me and I fall as though it's real. We have seven different endings – some with me winning, some with Rocky winning. Some segments we fake so good they are left untouched by the editors."

Ali has a point with the knockout sequences, which are realistic enough. And there are moments where a fight hints at breaking out, especially in the 12th where Ali connects with a series of playful flicks that get a snorting Marciano swinging widely. Mostly, though, the action was sloppy and forgettable.

"I think it was Marciano who threw the first real punch," Woroner said later. "They had been fooling around when Marciano suddenly let one go to the midsection. Ali followed with a shot to the head. But the fighters respected each other and apologized for these slips. And afterwards, Ali commented that Marciano had surprised him."

A friendship was forged outside the ring. Marciano, the bashful white man who served his country in the second world war, and Ali, the brash Afro-American draft dodger, found themselves getting on famously.

"Through all the fakery, something is happening between us," Ali wrote in his autobiography. "I feel closer to him than any white fighter in the trade. We talk fighter's talk in the way only friends can, blood talk, nitty-gritty talk. Our work is phoney but out friendship has become real."

Throughout filming Ali referred to Marciano as 'champ'. And in his autobiography he wrote: "Rocky was quiet, peaceful, humble, not cocky or boastful" adding that he "deserves his place as one of the greatest of the great heavyweights. Marciano, meanwhile, called Ali "the fastest man on wheels".

"But as the fraud came near an end, it was plain that neither of us, both heavyweight champions, liked the idea of being dramatized as defeated by the other – especially in a fake fight – and we were both on edge," admitted Ali. "One afternoon I unleashed a string of lightning-fast jabs that kept coming almost the entire round. Rocky was amazed and said: "I never seen a fighter with hands that fast."

The pair separated on good terms. A month later Marciano was dead when the three-seater plane he was travelling in from Chicago to Des Moines crashed into an oak tree in the middle of a cornfield. It was the evening before his 46th birthday.

THE AFTERMATH

Muhammad Ali was still barred from the ring when the film was screened in 1970. Photograph: Hulton Archive/Getty Images Chris Smith/Hulton Archive/Getty Images

On 20 January 1970, the Super Fight was shown as a one-time-only offering in 1,000 cinemas across the United States and a further 500 in Canada, Mexico and Europe. The result was "more closely guarded than the gold in Fort Knox," according to Time magazine. But some sniffed the future in the prevailing wind.

As Arnold Davis, the Philadelphia Inquirer reporter, told Ali: "That computer is no fool. You won't submit to White America's old image of black fighters, you won't even submit to White America's army. You're barred from the ring, stripped of the title, and on the other hand here is the real White Hope, the undefeated world heavyweight hero of the post-Joe Louis days … every self-respecting made-in-America computer knows how to add that up.

"You know what they want?" he added. "They want your ass whipped in public, knocked down, ripped, stomped, clubbed, pulverised, and not just by anybody, but by a real Great White Hope. We need Marciano to be able to club you into submission. They'll dig up the old heroes to say we had real red-blooded white men in those days that could handle niggers like this. A white ghost against a black ghost … Fantasy – but a lot of people live on fantasy. The end is supposed to be a mystery? To whom? Marciano will beat you bloody. And it will sell like hell in South Africa, to say nothing of Indiana and Alabama."

Others did more than smell the result in advance; they knew it. As Skehan put it: "One thing is certain: Rocky never thought he would lose. He had refused millions to make a comeback in the ring. There was no way he would risk losing a fight to a computer for a few thousand dollars." Just before Marciano died, just three weeks after filming, his brother Peter asked him: "How do you think you'll do in that fight?" "I'm a winner in 13," said Marciano, grinning.

After the plane crash, Peter phoned Woroner, concerned that the end would be changed. He needn't have worried: the result was exactly as his brother had forecast. During the 'fight' Marciano was bloodied, put down, and behind on points before coming back to win by knockout in the 13th round – an unimaginative regurgitation of his first championship bout with Jersey Joe Walcott.

Ali watched the fight in a crowded Philadelphia picture house; saw his left arm sagging on the middle rope as Marciano lifted his hands in celebration as the computer delivered its verdict: "Rocky Marciano wins by KO in 57 seconds. Knockout came on a combination of two rights and a left hook. Muhammad Ali though game could not withstand Marciano's final attack. Ali did not land a single effective punch this round." And he felt shame.

"I saw myself on the ropes being destroyed by Marciano, in one of the 'artistic' endings few actors could equal," he wrote. "But some people thought it was real. Some sat stone-still, some booed and yelled, some cried … I felt like I had disappointed millions all over the world. It left me ashamed of what I had been doing. I had gone over the country promoting the series as fair and accurate, especially the Marciano v Ali show."

His trainer Angelo Dundee was more sanguine. "To err is a machine," he joked.

Why did people believe the whole grand sham? Partly because they wanted to, of course. But this was also the era when man shot for the stars, and moonwalking was a reality not a dance. Technology was taking on all-comers and winning. Its dimensions were uncertain, its boundaries unclear – perhaps using it to 'solve' sporting hypotheticals wasn't so far-fetched.

Of course the Super Fight didn't settle the debate. It merely reset it. It matters little, but in a hypothetical Ali v Marciano encounter, most would make a prime Ali – the Ali that dismantled Cleveland Williams, before inactivity snatched much of the skip and slip from his legs – a strong favourite. But Marciano would have had a puncher's chance. And he certainly was a puncher.

Speaking to Howard Cossell on the Wild World of Sports in 1976, Ali paid his friend and acting partner a generous tribute, saying: "Ooh he hit hard … But I truly think on my best day and his best day I would have beaten him, probably not knocked him out. I think he was better than Joe Frazier, put it that way. And you know what Joe Frazier did to me.

"He wasn't as great as me, he wasn't as beautiful as me – everybody know that," he added. "But I don't know whether I could have beaten him with his style of boxing. He could have outpointed me, he could have knocked me down. I did a computer fight with him when he was an old man and just pretending and my arms were sore just from joking with him."

After the film was shown, Ali called the Super Fight "a sham" and "a Hollywood fake" on the Dick Cavett show. He was right. Even so, Woroner responded with a $2m lawsuit, claiming that another computer fight – this time involving Sugar Ray Robinson and the Frenchman Marcel Cerdan – had fallen through because Sugar Ray had lost faith in the computer.

Soon everyone else had too. In September 1970, the NCR 315 predicted that Joe Frazier would suffer a six-round defeat to Bob Foster in their forthcoming heavyweight title fight. No one else did, for Frazier was 21lbs heavier and swarming towards his prime. The fight was a mismatch: Frazier stalked his prey, before connecting with a left hook of such velocity that it twisted Foster's body like a spinning top and injured his ankle, before knocking him out 49 seconds into the second round.

Reality had caught up with fantasy. Woroner's idea was bust. But, with the Super Fight having taken at least $2.5m in gate receipts, he walked away a rich man. Ali, meanwhile, was getting ready to head out of exile and fight for real again.